If you want to skip the history, go directly to the list of all secret routes I've found. I provide more background on some prominent ones in the 400s, 500s, 600s, 700s, 800s, and 900s.
"Secret" routes are stretches of state-maintained road given route numbers for tracking purposes. They are not signposted or shown on the official tourist map. The typical secret route could be a long exit ramp, a road leading to a state park, or a connector road to publicly numbered routes. Some route are even temporary: on the logs for a year or two during construction, then turned over to town maintenance.
Connecticut has over 200 secret routes; most are less than a mile long.
Secret routes are numbered in the 400s through 900s. Signed "non-secret" routes are numbered in the 300s and lower.
The official name for these routes are State (or special) service roads (SSR's), and State roads (SR's). As of the 1962 route reclassification, the DOT uses this numbering convention:
Even if a state district changes boundaries, the affected routes are not renumbered; that's why some towns have a mixture of routes.
If the "secret" route numbers were only known to DOT employees, they wouldn't be of much interest. Fortunately for road geeks, the secrets leak out now and then.
Connecticut DOT news releases often mention SR's and SSR's, which some newspapers obligingly print without considering why these routes never seem to be signposted.
The internal DOT maps of each town mark all routes, including secret ones, and commercial maps seem to pick a few up from there. Given Rand McNally, Arrow, and Gousha maps (pictured), you would really expect to find a Route 664 in Killingly; but you won't, because there are no signs for it.
Secret routes are not supposed to be signposted, but human fallibility dogs the DOT as much as other agencies. At least three secret routes have also seen the light of day, however briefly.
One was an SR 771 sign Tim Parry once sighted in Fairfield. (It's now part of non-secret Route 130.)
The second is SR 540 (Hatchett Hill Road), near its intersection with Route 187 in E. Granby. (In late 2004, the sign was taken down; it had been improperly specified by a consultant on an earlier project along the road.)
The third was the high-profile leak: a big green freeway guide sign in Vernon. If you drove I-86 (now I-84) there in the early 1980s, you might have seen the short-lived sign for Route 533, Tunnel Road. This was during the I-86 reconstruction and it appeared that someone goofed in the sign department. To a teenage roadgeek (me) this was amazing to see, and when it was corrected (to plain "Tunnel Road") I felt a brief wistful sense of loss: a sign that couldn't be allowed to stand because it was different.
The idea of secret routes with their own range of numbers seems to date back to the Great Renumbering in 1932. Numbers reached the 900s quite early. In the Route Reclassification Act of 1962, the state set up the geographical numbering system described above. Almost all secret routes were renumbered, save for a lucky few, like SR 509, that happened to exist in the right area to fit the new convention.
Some towns contested state decisions on certain roads, and some routes remained under arbitration for several years. Kent's Macedonia Road, for example, was slated to revert to the town in 1962; however, it remained SR 477 until 1987.
Some routes in the 1920s were not signposted either; however, their numbers were mixed in with signed routes, instead of segregated to a particular range.
More than 400 different secret routes have been defined over the years. As of 2009, 225 still exist; the newest, introduced in 2000, is SR 661 in Windham, for the new Thread City Crossing bridge. My list has mileages, short descriptions, and history where available.
You can still find secret routes on maps, but the detective work can sometimes be daunting. The Arrow regional maps usually show the route numbers, and you'll see the occasional leak in national maps and atlases. (See the SR 664 example above.)
The official state tourist maps have never shown secret route numbers, but often showed where the routes went, since they are state maintained roads. The 1940s maps show the routes (but no route numbers); they're the roads drawn red without route numbers. Since the early 1970s, all official maps have done this (see figure). Any unnumbered red road is a secret route.
I've collected these routes from official Connecticut highway logs, town maps, state highway department documents, and commercial maps that have blown their cover. Before I had found these sources, however, Neil Kelly provided me dozens of street names from maps he found, and Paul Schlictman added "dates spotted" for several other routes. Hats off to them both.
Recent assembly acts have called for adding more local roads into the state highway system; most of these small roads would have become secret routes if actually admitted. Here's a list:
Secret roads are hard to uncover, especially those prior to 1963. Here's a list of reported "state roads" of the past, unfortunately without numbers:
For information about specific secret routes, see the complete list (terse entries; mileage, location, and dates), and more details for the more prominent routes.